Further progress has been achieved on the characterization of the antigen identified by the MAb, LIP-6, which is expressed by most bone marrow cells, pre-B and B cells and splenic macrophages but absent on mature T cells. Studies of a LIP-6+ Ly-5+ B220- bone marrow population show that it is 10-fold enriched for B cell CFU and 20-fold enriched with the addition of IL7, over whole bone marrow. Preliminary experiments suggest that this antigen may be closely associated with the B lineage antigen ThB and the IL5 receptor. LIP-6 can stimulate the in vitro proliferation of normal splenic B cells, maximally after 48 hrs. We have recently generated an anti-LIP-6 MAb of the IgG class, which will be useful for further biochemical and molecular characterization. More recent studies have focused on an antigen, Ly-26, which has been only partially characterized. Initial studies show that it is expressed by B and macrophages lineages, and CD4+ cells but absent on CD8+ cells. It may also subdivide CD4+ cells into two populations. In addition, anti-Ly-26 blocks the binding of anti-Mac-1 (CD11b) antibodies to macrophages, suggesting a relationship between these antigens.